Voltage tracking device



April 13, 1965 E. K. FULLER 3,178,588

VOLTAGE TRACKING DEVICE Filed July 11. 1961 INVENTOR EWWDER {ULLER A 7' TORNE V United States Patent 3,178,588 VOLTAGE TRACKING DEVICE Evander K. Fuller, Greensboro, N.C., assignor to Western Electric Company, Incorporated, New York, N.Y., a corporation of New York Filed July 11, 1961, Ser. No. 125,922 1 Claim. (Cl. 307-885) This invention relates to a voltage tracking device and more particularly to a variable tracking voltage reference device.

In the tracking of voltage reference signals the translation of these signals without distortion or interference is most important. Presently, variations in a power supply source utilized in translation circuits develops significant errors in the information of the reference signal. Also, the effect of atmospheric conditions, such as temperature, upon the operation of circuit components, develops erroneous signals.

One object of this invention is to provide a new and improved variable tracking voltage reference device.

Another object is the provision of a tracking circuit for translating a signal without normal interference from either variations in a power supply source or temperature.

Still another object is the provision of a tracking circuit having a pair of transistors wherein undesirable variations in a biasing source are substantially eliminated due to the connection arrangement and cooperation of the transistors.

With these and other objects in view, the present invention contemplates a Zener diode connected through resistance and across an emitter and a base of a first transistor wherein a collector and the base of the first transistor are connected to a base and an emitter of a second transistor. The emitter and a collector of the second transistor are connected to a pair of output terminals. A DC. power supply is connected through a resistor network to the collectors of both transistors and to the base of the second transistor.

The first transistor is normally conducting and providing a controlling potential from the collector of the first transistor to the base of the second transistor which is also conducting normally. A DC. voltage appears across the output terminals of opposite polarity but, representative of the reference voltage across the Zener diode. A varying signal may be applied to the emitter of the first transistor through a resistor. A signal will then appear across the output terminals inverted but representative of the input signal applied to the emitter of the first transistor.

Due to the arrangement of the transistors and resistor network with respect to the DC. power supply, any variations appearing in the power supply will vary the current drawn by the base and the collector of the second transis tor, and hence will vary the conduction of the second transistor, thereby maintaining the voltage output of the second transistor at a level representative of the input signal. Therefore, the variations present in the DC. power supply do not appear in the voltage appearing at the output terminals.

Additionally, any undesirable temperature conditions affecting the operating of the transistors will produce similar results upon the operation of the transistors, as is experienced from the varying DC. power supply, thereby preventing any undesirable results from appearing in the output of the signal being tracked.

Other objects and advantages of the invention will become apparent by reference to the following detailed description when read in conjunction with the accompanying drawing wherein the figure is a schematic diagram of a variable tracking voltage circuit embodying the principles of the invention.

There is shown in the figure a Zener diode 11 having a first terminal connected through a resistor 12 to a high negative potential power supply 13. Additionally, the first terminal of the Zener diode 11 is also connected through a rheostat 14 and a resistor 15 to an emitter 16 of an NPN transistor 17. A second terminal of the Zener diode 11 is conected to a base 18 of the transistor 17 and further to an emitter 19 of a second NPN transistor 21 and to an output terminal C. Further, an input terminal A is connected to the emitter 16 of the transistor 17 through a resistor 20 While a second input terminal B is connected to the second terminal of the Zener diode 11. A collector 22 of the transistor 17 is connected to a base 23 of the transistor 21 and through a resistive network to a positive power supply 24 wherein the resistive network consists of a series combination of a variable resistor 25 and a fixed resistor 26 in parallel with a fixed resistor 27. A collector 28 of the transistor 21 is connected to a junction 29 between the resistors 25 and 26 and is also connected to an output terminal D. The common lead 31 betweenterminals B and C is at a ground potential or reference level potential for the circuit.

In operation of the tracking circuit, the lead 31 between terminals B and C, as previously discussed, serves as a reference level. A negative potential, as supplied by power source 13, is developed across the Zener diode 11 and coupled to the emitter 16 with respect to the base 18 of transistor 17 while a positive potential provided by power supply 24 exists on the collector 22 with respect to the base 18 of the transistor. Under these biasing conditions, transistor 17 is rendered conductive.

Examining now the operation of transistor 21, it is seen that as transistor 17 conducts, the collector 22 will draw a certain amount of current from the positive power sup ply 24. Due to the voltage divider arrangement of resistors 25, 26, and 27, the voltage existing at point 29, and hence on collector 28, is of a higher positive value than that existing on the base 23 of transistor 21. Since the emitter 19 is connected to the common lead 31, as previously described, transistor 21 will now conduct. Due to the ohmic value of resistor 27, the potential appearing on the collector 22 of transistor 17, and hence the base 23 of transistor 21, will be of a very minute positive magnitude compared to the positive potential of power supply 24. This small potential is controlled by the collector of the first transistor in a manner that maintains a DC. voltage on the output terminals that is precisely related to the reference voltage across the Zener diode. The exact value of the output DC. voltage may be varied with resistor 25.

As a varying signal is applied to the input terminals A and B, the developed varying signal is applied to the emitter 16 and the base 18 of the normally conducting transistor 17 through the resistor 20. It will be noted that the power supply 13 provides a negative potential across the Zener diode 11 and on the emitter 16 at all times during the conduction of the transistor. Hence any developed varying signal will appear to vary about a Zener diode 11. Therefore, the conduction of transistor 17 will vary proportionately with the variation of the developed varying signal.

As the potential at terminal A appears to become less negative, the transistor 17 is biased into a lower state of conduction. Hence, the current drawn by collector 22 from power supply 24 will be of a lower value than that of the previous state. The base 23 of transistor 21 will now draw the current previously drawn by the collector 22 of the transistor 17. The increased current of base 23 conditions the collector 28 of transistor 21 to draw more current, thereby rendering the potential of the collector 28 at a less positive value than was previously present.

Thus it will be noted that for a positive approaching potential applied at the input terminals A and B, a negative approaching potential will appear at output terminals C and D thereby providing an inverted duplicate of the input signal at the output terminals.

In the event there is a variation of potential in the positive power supply 24, it will be desirable to maintain the potential existing at point 29, and hence collector 28 and output terminal D, at a constant value. If the magnitude of the positive potential of power supply 24 should decrease to a lesser positive value, it would appear that the conduction of transistors 17 and 21 would be affected. However, due to the minute potential required on the collector 22 of transistor 17, any large change in the potential of power supply 24 will theoretically produce no change in the collector potential of transistor 17. Hence, transistor 17 appears as a constant current generator, thereby drawing a given amount of current regardless of any variation in power supply 24. Since the collector current drawn by transistor 17 remains theoretically constant, the current drawn by base 23 of transistor 21 will be less than that drawn before the decrease of potential in power supply 24. The decrease in base current of transistor 21, renders the transistor in a less conductive state, thereby theoretically presenting a larger emitter-to-collector resistance. Since the circuit consisting of resistor 26 and the emitter-collector circuit of transistor 21 are connected between power supply 24 and the reference level 31, the decrease in the positive value of the positive potential of the power supply 24 will decrease, in a less positive fashion, the magnitude of the current through this circuit. However, as previously noted, the base 23 of transistor 21 is also connected to the positive power supply 24 through the resistor 27, thereby decreasing the base current drawn from power supply 24 upon the previously mentioned decrease in the positive magnitude of the power supply. Therefore, the emitter-collector circuit of transistor 21 appears as a varying resistance during the period when the base current is being reduced, and in effect varies the potential appearing at the collector 28. As previously mentioned, the current through the emittercollector circuit is also varying. However, the variations of the emitter-collector resistance and current are of such a proportion that the potential appearing on collector 28 remains constant throughout large changes in magnitude of the potential from power supply 24, thereby rendering ineffective any variations in the power supply upon a developed varying signal which is being translated from the input to the output.

Thus it is seen that if the potential of power supply 24 should vary to a more positive value, the current drawn by collector 22 of transistor 17 would again remain theoretically constant. However, the base 23 of transistor 21 would draw additional current, thereby increasing the conduction of transistor 21. As transistor 21 increases in conduction, the emitter-collector resistance is lowered, thus lowering the positive potential appearing on collector 28. However, the emitter-collector current has increased due to the change in emitter-collector resistance. As previously stated, the varying resistance and current is of such a proportion so as to maintain the potential of collector 28 constant throughout any variations in the power supply 24.

It is to be noted that the ohmic value of resistor 27 is 4 large so as to maintain an extremely low operating potential on the collector 22 of transistor 17. In this manner the current drawn by the collector 22 will remain theoretically constant upon variations in the power supply 24.

In addition, it is common knowledge that temperature conditions affect the operation of a transistor. In the present case, however, the temperature will not affect the operation of transistor 17 due to the low collector bias required for operation. Also, in the event the temperature should affect transistor 21 in such a manner as to alter the currents of base 23 and collector 28, the output of transistor 21 is maintained constant in the same manner as previously described.

The resistor 25 is connected as previously described in order to vary the base-collector potential of the transistor 21, thereby controlling the magnitude of the output signal appearing across terminals C and D. Therefore, while the output signal is an inverted duplicate of the input signal, the magnitude of the output signal can be controlled.

Additionally, if the resistor 25 was adjusted such that the gain ratio of the input to the output was one to one, the circuit could function as a variable tracking reference of opposite polarity to the input reference. Also, the ability of the circuit to ignore voltage variations appearing at the collector 22 and the insensitivity of the circuit to temperature changes, renders the circuit useful as an A.C. or DC. computer-type amplifier in addition to the reference voltage applications.

It should be understood that the above-described arrangement of circuit components and construction of terminal parts are simply illustrative of an application of the principles of the invention, and many other modifications may be made without departing from the invention.

What is claimed is:

A reference tracking circuit comprising a first NPN and a second NPN transistor, each having a first and a second base, emitter, and collector, respectively, means for connecting an input terminal to said first emitter, a Zener diode having a first and a second terminal, an output network having a first and a second terminal, a voltage source, subject to variations, means for connecting said first terminal of said Zener diode to said first emitter, means for connecting said second terminal of said Zener diode to said first base, means for connecting said first base to said second emitter and said second terminal of said output network, means for connecting said first collector to said second base, a resistive network for interconnecting said voltage cource'with said first collector and said second base and said second collector for compensating the voitage variation in said voltage source, and means connecting said second collector through said resistive network to said voltage source and to said first terminal of said output network.

References Cited by the Examiner UNITED STATES PATENTS 2,751,550 6/56 Chase 30788.5 2,816,964 12/57 Geacoletto 33040 2,879,410 3/59 Loeb 307-88.5 2,953,737 9/60 Beach et a1. 307-885 3,007,102 10/61 Kennedy 30788.5 3,080,528 3/63 Davidson 33025 OTHER REFERENCES Electronics article October 9, 1959, Constant-Current- Coupled Transistor Power Supply, pages 60-61.

ARTHUR GAUSS, Primary Examiner. ROY LAKE, Examiner. 

